In the past, as controls for the charging of a lithium ion secondary battery, a charging voltage control, a charging current control, a control which enables the charging only within a predetermined temperature range, and the like, have been incorporated in a battery charger. In a battery pack in which a lithium ion secondary battery, etc., is incorporated, a control which detects the voltage of a battery and interrupts the charging when the voltage is equal to or greater than a predetermined voltage value is provided as a provision for the case where charging voltage control does not work. The above-described charging interruption control provided in the battery pack is termed as overcharge protection.
Furthermore, a control has been performed in which the temperature of a battery in the above-described battery pack is measured by a thermistor, an output of which is connected to a battery charger, and in the battery charger, the charging is enabled only within a predetermined temperature range.
On the other hand, conventionally, as the control associated with the discharging of the lithium ion secondary battery, etc., a control which interrupts the discharging when a load short-circuit occurs, a control which detects an excessive rush current during the discharging and interrupts the discharging, and a control which detects a constant current during the discharging and interrupts the discharging have been performed. These controls are generically referred to as an overcurrent protection.
Controls at the battery pack side, such as the above-described overcharge protection, the thermistor control, and the overcurrent protection, are generically referred to as safety circuits.
The above-described overcurrent protection is one of the protection functions for a battery such as a lithium ion secondary battery, etc. However, in many cases, in preparation for a case where this protection does not work, a PTC element having a ring shape is installed in the lithium ion secondary battery itself, or a PTC element is provided in the battery pack.
The PTC (Positive Temperature Coefficient) element is an element such that the resistance value is low during a normal operation, but abruptly increases when heated by itself or by an ambient temperature. This element has been used for overcurrent/heatup protection.
A battery pack which uses a lithium ion secondary battery and the secondary battery control circuit (safety circuit) is described in, for example, Japanese Laid-Open Publication No. 10-275612.
However, in the above-described conventional structure, in the case where a PTC element having a ring shape which has been installed in a lithium ion secondary battery itself is activated, an expansion force in a direction of the thickness of the PTC element is affected in a direction in which a caulking portion of a battery sealing portion is opened. This has been a cause for leakage of electrolytic solution. Furthermore, there has been a concern that the deterioration in a life cycle or the swelling of the secondary battery which results from the fully-charged lithium ion secondary battery, etc., being left in a high temperature condition.
Furthermore, there has been a concern that circuit malfunction, corrosion, or problems due to hydrofluoric acid generated by adding water to an electrolytic solution may be caused in the case where a liquid infiltrates into a secondary battery such as a lithium ion secondary battery or into a battery pack in which the above-described secondary battery is installed, or in the case where an electrolytic solution inside the battery is leaked.
In general, the main component of an electrolytic solution which is used for a lithium ion secondary battery is lithium hexafluorophosphate.
The chemical reaction caused in the case where water is infiltrated into lithium hexafluorophosphate is represented by expression (1-1). By this chemical reaction, hydrofluoric acid (i.e., a very strong acid) is generated.
 LiPOF6 (lithium hexafluorophosphate)+H2O→LiPOF42HF (water is present)→LiF+POF3↑+2HF↑(after decomposition)  (1-1)
The present invention solves such conventional problems. An objective of the present invention is to provide a secondary battery control circuit which can reliably control charging/discharging without activating a PTC element, which reduces deterioration in life cycle and the occurrence of swelling of the battery, and which can be installed in a small space section in a battery pack.